Method of reducing cobaltic ammine salt



United States Patent 2,749,235 IVIETHOD 0F REDUCSIZISI COBALTIC AMMINE Tuhin Kumar Roy, Elizabeth, N.'J., assignor to Chemical Construction Corporation, New York, N. Y., a corporation of Delaware No Drawing. Application September 25, 1953, Serial No. 382,467

3 Claims. (Cl-. 75-119) The invention relates. to the production of cobaltmetal. More particularly it is concerned withtheproduction of cobalt metal powder from a cobaltic salt. Still. more particularly it relates to the production of cobalt metal powder by reduction of a cobaltic ammine sulfate.

Cobalt and nickel metals and their naturally occurring minerals are very similar to one another in many physical and chemical properties. Minerals of these metals frequently occur together in natural deposits and cannot be readily separated by ordinary'mineral dressing methods. Moreover, in the conventional metallurgy of either nickel or cobalt, the presencein either of relatively large quantities of the other has always been a serious and difiicult problem in the preparation of either as a relatively pure metal.

For example, when small amounts of nickel are present in cobalt mineral concentrates they follow through into the solutions formed in producing cobalt. This nickel usually either is discarded at considerable cost or recovered with the cobalt; In the latter case thenickel is generally considered an impurity'in-the cobalt metal and the producer is generally not paid for it. Although in some cases, certain nickel and'cobalt mixed metals are marketable as such, usually the price is below the combined value of the pure metals if in separate products.

There has, therefore, alwaysbeen a need for a process for the efficient separationof cobalt and nickel into "separate metallic products.

In recent years, several processes have been proposed for the separation of cobalt and nickel values. One of the apparently most promising of them utilizes the preferential oxidation of cobaltous values in an ammoniacal solution to precipitate a cobaltic ammine away from a nickelous salt solution. Under proper conditions of temperature and pressure there is obtained in this process cobaltic ammine salt which is free of nickel. This salt is considered to be the cobaltic hexammine salt. It is usually but not necessarily formed as the sulfate.

This method is ideally adapted to the production of cobaltic salt substantially free from contamination by nickel. However, it is diflicult to obtain substantially pure cobalt metal from cobaltic ammine salt in that it is difiicultly soluble in strong ammoniacal solution and but little more soluble in hot water, dilute ammonium hydroxide and dilute acids. These solutions are not suitable for the desirable step of reducing cobalt metal from an aqueous salt solution. Thus even though the cobaltic salt can be obtained in a relatively pure form there still remained the problem of obtaining cobalt metal therefrom.

It is, therefore, the object of this invention to overcome this difiiculty and present a simple process for the obtaining of cobalt metal from a cobaltic hexammine salt in a simple and eflicient manner.

Surprisingly, this object has been achieved in an unusually straightforward manner. The cobaltic hexammine salt, however produced, is collected and, if necessary, dried. The dry salt is then treated to convert it 2,749,235 Patented June 5, 1956 Whatever the mechanism, the resultantproduct mixture.v after treatment is dissolved, or at least slurried, in water and is then subjected to reduction. This may be by electrodeposition if so desired. However, its primary purpose is to enable reduction with a suitable reducing gas at elevated temperature and pressure according. to any desired reduction practice. Under optimum conditions, a cobalt metal powder is produced having a purity as high as 99% or. better. Since this is well above the grade which is acceptable commercially, it is a highly desirable product. The economic loss of potential nickel product is minimized.

While this would appear to complicate the process by introducing an additional step, this is not a commercial disadvantage. The resultant overall treatment is' far simpler, more eifective andmore economical than any previously attempted method ofv converting the ammine salt directly to metal withoutthe: intermediate step.

Heating, or roasting of the cobaltic ammine salt is accomplished in the range from about 275 F. to about 950 F. Temperatures. lower than. about 275 F. are not really. practical without excessively large apparatus; Decomposition. of, cobaltous. material tendsto set in at above. about 9 50" F. Actually the preferred temperature range is.450."6 001 F.

The heating or roastingoperation maybe carried out inany convenient apparatus. However, since one of'the advantages of the present. invention resides in the convenient recovery of the ammonia-oxygen olf-gases,.the apparatus should preferably be such that the ammoniaoxygen off-gas can be readily collected and reused.

The cobalt'ous'salt mixture resulting from heating the cobaltic ammine salt is next mixed with water to form a solution or slurry. The relative amounts of water and salt mixture are not critical. It is an advantage of the gas reduction process that not all of the salt must be dissolved. Rather it is preferable economically to use enough of the salt mixture to water to form a thin slurry rather than a solution. This results in a saving of sensible heat in the subsequent steps and permits better utilization of the capacity of the reduction vessel. A good practice and perhaps the preferred concentration runs about 12 parts of water per part of cobalt. If the other conditions are adjusted cobalt powder is reducible from any stirrable slurry. However, if too small a portion of the cobalt salt is in solution at any one time, reaction may be too slow.

Resultant aqueous slurry is then heated directly to the reduction temperature, preferably 350-475 F. before or after introduction into a suitable pressure vessel. The system is then given a positive partial pressure of a reducing gas, preferably hydrogen and the mixture is stirred at reduction temperature until optimum cobalt metal precipitation is obtained.

At terminal conditions the pH of the mixture should not be below about 2-4 and preferably should be above about pH 6. Accordingly ammonia or other neutralizing agent is introduced as necessary to maintain this condition. About 2 to about 6 mols of ammonia per mol cobalt may be added.

The preferred pressure range is from about 400 to about 900 p. s. i. g. although results can be obtained at any pressure equal to or above that equivalent to the combined vapor pressures of the solution plus the reducing gas partial pressure. However, unduly excessive total pressures do not produce results commensurate with the increased cost for necessary apparatus. The cobalt powder is then recovered as by decantation, or its equivalent, washed and dried.

During reduction, the total pressure on the reduction system is usually that produced by the various partial pressures of the solution plus the pressure of the reducing gas. In general, in the preferred practice the total pressure may also be defined as being from about 50200 p. s. i. g. of a reducing gas above the total vapor pressures of the liquid at the reduction temperature. Higher pressures are not needed. Of the reducing gases commercially available, those which are sulfur-free should be used. Practically, these are hydrogen, carbon monoxide and mixtures of them. Hydrogen alone, where available, is preferred to avoid carbonyl formation.

The end point of the reduction can be determined by tests indicating the substantial absence of Co++ ion. Reduction time generally runs about 15-30 minutes and in most cases less than two hours. The reduction is carried out generally along lines familiar to those skilled in the art.

The following examples are intended as illustrative of the invention and not necessarily by way of limitation. Except as noted all parts are by weight.

Example 1 An orange colored cobaltic hexammine sulfate obtained in the preferential oxidation of an ammoniacal nickelcobalt solution is used as the feed. A sample of this salt is heated to 480-580 F. The salt is observed to decompose rapidly with evolution of ammonia and oxygen, leaving a pinkish crystalline mass. This pink colored product on dissolution in water does not give any reaction of cobaltic ion but responds to the test for cobaltous sulfate and ammonium ions. The salt analyzes: Co-26%, SO4-65% and NH3-7%. From the above experiment it is concluded that the hexammine salt, on heating breaks up into cobaltous sulfate and ammonium sulfate.

To a solution of such resultant pinkish crystals containing 1 mol of C0504 and 0.5 mole of (NH4)2SO4 is added three mols of NHa, and 50 parts of pure cobalt metal seed powder running about 40-100 microns in diameter. Resultant slurry is heated to 400 F. and pressurized with hydrogen to 800 p. s. i. g. in a stirred autoclave and treatment continued for about one hour. Resultant powder is collected by decantation, washed and dried. It is a highly pure cobalt powder assaying about 99% cobalt.

Example 2 Another sample of the same cobaltic hexammine sulfate is heated as in Example 1 and is then mixed with water to make a solution containing 1 mol of C080; and 0.5 mol of (NH4)2SO4. The solution is heated in an autoclave to 400 F. and again pressurized with hydrogen to about 800 p. s. i. g. Ammonia is slowly pumped into the autoclave to keep the pH of the solution between 2.2-6. After /2 hour ammonia is pumped in faster in order to make the pH rise above 6. The reaction is continued until reduction is complete, in about thirty minutes. The cobalt metal powder is then collected as in Example 1 and found to be of comparable quality.

I claim:

1. A process for producing elemental cobalt metal from cobaltic hcxammine sulfate which comprises, heat ing cobaltic hexammine sulfate to a temperature of from about 275 to about 950 F., forming an aqueous ammoniacal liquor of the decomposition product in which at least a part thereof is in solution, and subjecting said liquor to the action of a reducing gas at above about 300 F. and an overpressure of reducing gas greater than about p. s. i. g.

2. A process according to claim 1 in which the cobaltic hexammine sulfate is heated to a temperature of 450600 F. and the temperature maintained until evolution of gas substantially ceases.

3. A process according to claim 2 in which the by drogen ion concentration of the liquor formed of the decomposition product is initially adjusted to and maintained during reduction at a pH greater than about 2.

References Cited in the file of this patent UNITED STATES PATENTS 

1. A PROCESS FOR PRODUCING ELEMENTAL COBALT METAL FROM COBALTIC HEXAMMINE SULFATE WHICH COMPRISES, HEATING COBALTIC HEXAMMINE SULFATE TO A TEMPERATURE OF FROM ABOUT 275* TO ABOUT 950* F., FORMING AN AQUEOUS AMMONIACAL LIQUOR OF THE DECOMPOSITION PRODUCT IN WHICH AT LEAST A PART THEREOF IS IN SOLUTION, AND SUBJECTING SAID LIQUOR TO THE ACTION OF A REDUCING GAS AT ABOVE ABOUT 300* F. AND AN OVERPRESSURE OF REDUCING GAS GREATER THAN ABOUT 50 P. S. I. G. 